Garbage collection question

[Vivek Raj]

Chapter - 3 Assignment, Self Test Q#11 from K&B SCJP 1.6

class Beta{}
class Alpha{
static Beta b1;
Beta b2;
}

public class Tester {
public static void main(String[] args){
Beta b1 = new Beta();       Beta b2 = new Beta(); 
Alpha a1 = new Alpha();    Alpha a2 = new Alpha();

a1.b1 = b1;
a1.b2 = b1;
a2.b2 = b2;
a1=null; b1=null; b2=null;
//How many objects will be eligible for garbage collection at this line?
}
}

Answer is 1 object will be eligible for GC.
However I can see 2 eligible objects, one is object a1 itself and the other is object a1.b2 which also we cannot refer after setting a1 = null.

Please clarify.

[Stuart A. Burkett]

Vivek Raj wrote:However I can see 2 eligible objects, one is object a1 itself and the other is object a1.b2 which also we cannot refer after setting a1 = null.

a1 and a1.b2 are not objects, they are references to objects. An object is only eligible for garbage collection when there are no reachable references to it.
You're right that when a1 is set to null, any object referred to by a1.b2 might also be eligible for garbage collection. So if it isn't, there must be another reference to the same object - can you see what it is ? Have a look at line 12.

[Vivek Raj]

I got the point.

So if w change line4 to
Beta b2 = new Beta();

then there would be 3 objects for GC.